The main spectrometer on Mars Express, called OMEGA, has now returned a planet-wide data set, and Bibring et al. (p. 400) have used these results in combination with related observations by other Mars orbiters and the two rovers to reconstruct the history of water alteration on Mars. Hydrous minerals are abundant only in the oldest rocks; sulfur-rich minerals are present in some younger rocks, but more recent alteration is anhydrous. This record implies that there was likely surface water only early in Mars history, which gave way to more ephemeral acidic alteration. Water-rock interactions are not apparent after about 3.5 billion years ago.